Synthesis of a Heteropolytungstate and its Use in Outer-Sphere Redox Kinetics An Inorganic Chemistry Laboratory Experiment Frank Walmsley Trinity University, San Antonio, TX 78212
An inorganic chemistry experiment has been developed combining two topics usually included in inorganic chemistry courses: heteropoly ions and mechanisms of oxidation-reduction reactions of transition metal ions. These topics were chosen because suitable experiments are not available that include these topics either singly or together. Iso~olv. and hetero~olvions(now called ~olvoxometalates) . " remain a myswrytr~~a~chemistrystudentsrandchemists, althoueh the a~~lication of fundamental ~ r i n c d e has s Dcrmitted-the na&e of their structures to be kerstood-(1). Students sometimes enwunter the phosphomolybdate ion in the spedrophotometric analysis of either phosphate or molybdate and in development of TLC plates in biochemistry Hoth heteropolymolyl~atrsand tung&ws have applicatio& as catalysts in a wriety of reactions. Other applications include 0; carriers and antiviral agents. A recent review includes advances in the structures of these materials and in their uses (2). Oxidation-reduction reactions of coordination compounds are usually described in terms of either outersphere or inner-sphere mechanisms. The ion CoW1z040b, which contains Co(III), is an excellent oxidizing agent (3) forming CoW120406 as its product. Both CoW1zOqob and CoWDOaO6have what is called the Keggin structure as shown in Figure 1. It has been shown that the Co%lCo3+ redox couple in CoWlzOao61CoW1z0405~ is similar to that in normal complexes (4). In the case of an inner-sphere mechanism of a redox reaction, there is usually atom (or group) transfer as the means by which the electronis transferred. Since Co(II1) in CoWlZOaobis buried within the oxoanion
.
Figure 1. The Keggin structure of the heteropolytungstate.Atungsten is at the center of each octahedron, an oxygen at each corner, and a cobalt in the center of the structure tetrahedrailv coordinated throuah oxygens to four interconnected W3010 groups; two of the four oiygens coordinated to the cobalt and two facesof the tetrahedron are visible.
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Journal of Chemical Education
structure, it has no transferrable groups and cannot have any atom or group transferred to it. As a result, the mechanism of oxidation by CoWlzOaobis expected to be outersphere. Upon reduction, the ion remains intact; the only change is in the oxidation number of the cobalt and the concomitant change in the ion charge. The mechanisms of oxidation of a number of substances by have been reported (5-13). In every case the reaction is first order in CoW120406-and first order in reducing agent. Also, the evidence strongly supports an outer-sphere mechanism in every case. The synthesis of the hetero~olvtunestateand its use as an oxidizine aeent with three r;duc~%~agents is reported here: SEN=(^), HNOz (61, and HPHzOz
(n.'
Experimental The ~otassiumsalt & C O W ~O readilv H~O .- ~."~ O ~ ~ . is . svn. thesized in one laboratory period by one of the methods of Baker (141if intermediates are not ourified. Dissolve 19.8 g (0.06mol) of NazW04.2Hz0in 40 &L ofwater. Add 3 -3.5 mL of glacial acetic acid and check, with pHydrion paper, to be sure the pH is 6.5 to 7.5. Dissolve 2.5 g (0.01 mol) of Co(CzH~0&4HzO is 12-13 mL of water to which two drops of glacial acetic acid has been added. Heat the Na2W04solution to near boiling and add the Co(CzHa02)2solution to it all a t once with stirring. Boil the resulting mixture gently for about 15 min. Then add 13.0 g of KC1 to the boiling solution. Cool to room temperature and separate the precipitate by filtration on a Biichner funnel. Use some of the filtrate to wash the remaining solid from the beaker onto the filter. Remove excess moisture on filter paper. Add about 25 g of the solid just prepared to 40 mL of 2 M HzS04and heat gently for a few minutes. Filter the solution to remove any undissolved solid. This solution should contain the CoW120406ion. Heat the solution to boiling and add, with stirring, solid KzS2O8in about 0.5 g increments until the solution turns to a gold wlor (about 10 g). Continue heating for 5 min. to decompose any excess KzSzOs. Cool in an ice bath and filter the precipitate using a Biichner funnel. The crystals should be reasonably pure & C O W ~ ~ O ~ ~ ~Recrystallization ~OH~O. using no more than 60 mL of water is possible but not required. The kinetics are followed spectrophotometricallyat 388 ~ 0in~the visible renm, the maximum of the C o W ~ 0 ion gion of the spectrum. In each case it is possible to determine the reaction order in both reducing agent and in CoWUOqobwith a minimum of two runs if the concentrations are carefully chosen although the use of at least four runs is preferable in order to average errors and to illustrate the technique for handling this type of data. The initial COW^^^^^^ concentration is the same in each run and the 'A set of instructions given to students that includes amounts of materials and typical student results can be obtained fromthe author.
-1.60 -1.80
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T h ~ o c y a n a t eK i n e t i c s
Thiocyanate K i n e t i c s 4.00
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I
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m
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-
-2.60 --2.80 --3.00
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7
0
slope = 0.625 L s-' m d ' .
0.50 50
100
150
200
250
300
350
400
Tinids Figure 2. First order rate law plots fromstudent data forfourdifferent concentrations of the reductant, SCh-. The negative valJes of the slopes of tne ihnes are the k& va ues.
initial reductant concentration is varied with an excess of reductant in every case. The solvents used are 1.0 M NaN03 for SCN-; 1.0 M NaCl and 0.10 M HC1 for HNOZ; and 1.0 M NaCl and 1.0 M HCl for HPH202.Usinrr the appropriate solvent, solutions are prepared asfollow% COW^^^^^^ -4 x lo4 M for HNOz and SCN-; -2 x lo" M for HPH202.The actual concentration can be measured from the absorbance at 388 nm where the absorptivity is 1150 L mol-' cn-'. NaN02: 2 x lo3 M,4 x 10" M, 6 x lo" M, and 8 x lo" M.NaPH2O2.H20:1.6 M, 2.4 M, 3.2 M, and 4.0 M. NaSCN: 4 x 10" M, 6 x lo3 M,8 x 10" M,and 12 x lo3 M. Equal volumes of the oxidizing and reducing agents are mixed and absorbance measurements are begun as soon as possible, preferably within one minute. The actual initial concentrations will be oue-half those given above due to the dilution upon mixing the reactants. The reaction is followed s~ectm~hotometrica~~v at 388 nm and each run takes ~ l 2 ~ m i?he n . concentrations of oxidant and reductants are chosen to keep the time reasonably short and to keep the absorbance values measured less than one. Students need to record Aversus time values every 20 s. In addition, an absorbance value at time equals infinity is needed. This is determined after warminc the soluti& to drive the reaction to completion. The methGd by which data are obtained will vary with the capabilities of the spectmphotometer being used. Results and Discussion
The reducing agent is used in excess which gives a pseudo-first-orderreaction in the reducing agent. It is possible to determine the order in the oxidizing agent by plotting either wncentration, ln(concentration)or Uconcentration versus time. Since there is a residual absorbance at time equals infinity, absorbance values cannot be directly substituted for concentration;A -A- must be used. A set of student results for oxidation of SCN- is shown in Figure 2. The linearity of these plots shows the reaction to be first order in oxidant. By varying the concentration of the reducing agent and seeing how the hob varies, it is possible 2Phosphinicacid is sometimes written as the nonexistent H?P& and called hypophosphorous acid; commercial sources of chemicals call sodium phosphinate monohydrate sodium hypophosphite monohydrate. Phosphonic acid is sometimes written as the nonexistent H,P03 and called phosphorous acid. While hno of these acids are nonexistent, the isomers must be named correctly in order to correctly identify the parent acid of derivatives of the acid.
7
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
103[SCNWN F gure 3. Plot of the k,, valLes from F gure 1 versus reductant con-
centration. The positive s ope and lbnearty hndcate the order in reductant is one.
to determine the order in reducing agent. The order in reducing agent is determined by plotting k o b versus initial reductant concentration (or initial reductant concentration squared) and the overall rate constant determined from the slope of this line. The graph of the k,b values from Figure 2 is shown in Figure 3.This figure shows that the reaction is first order in reductant. The data also can be examined in a manner analogous to that of the method of initial rates included in most general chemistry texts. If the reaction is first order in reductant, the value of hob. should double when the concentration of reductant is doubled. The reductant concentrations are chosen to make it possible to observe this. The SCN- is oxidized to thiocyanogen, (SCNX, which reacts with excess SCN- to form (SCNI3-. Thiocyanate ion can be described as a pseudohalide ion, and its reactions here are analogous to those of iodide ion which is oxidized to Iz which reacts with excess 1- to form 13: The HNOz is oxidized to NO3 as might be expected. The phosphinic acid HPHzOz is oxidized to phosphonic acid HzPH03.' In the case of an outer-sphere mechanism between two anions, the rate is oRen dependent on the concentration of cations, especially the concentration of alkali metal cations. The cation; catalyze the reaction by providing a means to bring the two like-charged anions together to permit electron transfer. In order to isolate this effect, that could add another dimension to the experiment (and could be made a part of the experiment), all kinetic reactions have been carried out with a constant alkali metal ion concentration. Some reactions can be hydrogen-ion dependent. If the reductant is a weak acid or the salt of a weak acid, the reductunt could be present in two forms: the neutral molecule and the anion. The rate of reaction of these will not be the same because of the difference in charge. Thus, if both forms are present, there will be two competing reactions. This can be handled either by using a large concentration of hydrogen ion or by using a buffer.The systems studied here use a large hydrogen ion concentration. The use of HC1 and NaCl to adiust the pH and the alkali metal ion concentration are convenient but preclude a study of temperature dependence. At high temperatures, CoWlzOao" is a strong enough oxidizing agent to convert Cl- to Clz. Thus, for high temperature studies, a nonoxidizable anion such as NO3- must be used. Additional reducing agents have been studied and with proper adjustments may be suitable for use in this type of Volume 69 Number 11 November 1992
937
experiment: 1-in C H 3 0 H (aq) (8). Sz0z2 (9),~ 0 cysteine (111,C20a2 (IZ), and ascorbic acid (13).
(lo),L
3 ~ -
Literature Cited
Addendum
The original article on the SCN- system (5)identifies the product in the presence of excess SCN- as ( S C N ) 2 and proposes a mechanism based on this assumption. Then the authors purportedly determine the products of the reaction by using an excess of oxidizing agent as had been don, in a previous study (15).This must be incorrect for the kinetics study because the conditions are not the same and is not consistent with the kinetics results.
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Journal of Chemical Education
7. A&ko, G. A: lionsition Met C h m . 1990,15,421. I.:McAuley,A.%nsitionM#t 8. Blandarner,M.J.:Burgess,J.:Duce,PP:H~es,R. Chrn. 1982,7,10. 9.AX, M.;Sahq S. K; Banerjee, P. lndion J C h m . 1980,29A, 528. 10. ALM.: . . Saha. S.K:.Ban&ee.P. . . J. Chem Soc.Dalton %inns. l s W . 187. 11. Ayoko,G. A,; Olatunji, M.A.Polyhedmn 195S.2.577. 12. Sahs. S. K.; Ghosh. M. C.; Banenee, P J. Cham. Soc Dalton l i o n s . 1 W . 1301. 13. Pelizzetti, E.; Mentasti, E.; Ramaura, E. Inorg Chom. 1918, 17, 1181. Amjad, 2.; Bmdoviteh, J. C.; McAuley A Con. J Chen. 1911,55,3581. 14. Baker L.C. W.: McCutehean.T P J Am. Chem. Soc. 1956.78.4503. 15. Ng, l? T T;Henry P M. Can. J Cbm. 19T5.53.3319.
